1. Field of the Invention
This invention generally relates to the fabrication of integrated circuit (IC) devices, and more particularly, to an oxide interface on silicon and a method for forming the same using high-density plasma.
2. Description of the Related Art
FIG. 1 is a schematic of a stacked gate structure for a thin film transistor (prior art). The proper performance of IC devices depends, in part, on the characteristics of oxide layers within the device structure. A thin film transistor (TFT) will be used as an illustration, however, it is understood that the discussion applies to other IC devices as well. In FIG. 1, oxide layers form the gate insulator. Both the bulk characteristics of the gate insulator and the characteristics of the interface between the gate insulator and the silicon layer are very important for the operation of a TFT. For silicon devices, a good gate insulator film is silicon dioxide (SiO2), and a good method of forming a high quality SiO2 film with excellent bulk and interface characteristics is by thermal oxidation. For a TFT, thermal oxidation involves forming a layer of silicon over a diffusion barrier and substrate and heating the resulting stack structure to form a layer of SiO2 overlying the silicon layer. To produce an oxide layer at growth rates rapid enough to be economically practical, thermal oxidation typically is performed at temperatures between 800° C. and 1200° C. Only a limited number of substrate materials, for example, silicon, are compatible with the temperatures required for thermal oxidation.
FIG. 2 is a schematic drawing of a plasma enhanced chemical vapor deposition (PECVD) system (prior art). The use of substrate materials incompatible with the temperatures associated with thermal oxidation is of increasing interest. For example, improvements in liquid crystal display (LCD) technology create a need for high performance TFT driver components on transparent substrates such as glass and polymer. Unfortunately, the transparent substrates noted above are incompatible with the temperatures required for thermal oxidation. In fact, it is desirable to process these substrates at temperatures below 400° C. (hereafter referred to as low temperature). Unfortunately, the use of PECVD at low temperature results in higher impurity levels for oxide layers than are typical for thermal oxide or PECVD oxide formed at temperatures greater than 400° C. In addition, low temperature PECVD results in lower oxide deposition rates than are associated with PECVD at temperatures greater than 400° C. For typical low temperature PECVD oxide layers, characteristics such as refractive index, fixed oxide charge density, breakdown field strength, leakage current density, and interface trap density are all inferior to those for typical thermal oxide layers. For example, thermal oxide has a refractive index of 1.46, while low temperature PECVD oxide has a refractive index of less than 1.45. Modifying low temperature PECVD process parameters to increase deposition rates reduces the quality of the bulk and interface characteristics for the resulting oxide. The process in FIG. 2 uses capacitively coupled plasma. The high frequency power is directly connected to the top electrode and capacitively coupled to the bottom electrode. The two electrodes are therefore coupled, and it is not possible to independently control energy directed to the top and bottom electrodes. Therefore, any attempt to enhance the growth rate by increasing the high frequency power leads to an increase in the sheath potential which adversely affects oxide bulk and interface properties.
It would be advantageous if a low temperature process could form oxide layers with bulk and interface characteristics superior to oxide layers formed by low temperature methods such as PECVD.
It would be advantageous if a low temperature process could form oxide layers with bulk and interface characteristics approaching those for thermal oxide.
It would be advantageous if a low temperature process could deposit oxide at rates greater than those for low temperature methods such as PECVD.